"Be quick to hear, slow to speak and slow to anger (Jas 1:19)." That sounds like the same sort of thing you would read in Proverbs. But James is much more emphatic, "Let not many of you become teachers, my brethren, knowing that as such we will incur a stricter judgment ... If anyone does not stumble in what he says, he is a perfect man...(Jas 3:1-2)." Why do teachers incur a stricter judgement? Because they use a tool that has great power and with great power comes great responsibility. What is that power? Words.
Our words are not as powerful as the word of the Lord that can create and destroy the universe, but our words can destroy so many other things. Notice, "So also the tongue is a small part of the body, and yet it boasts of great things. See how great a forest is set aflame by such a small fire (Jas 3:5)!"
This is why James says, "So speak and so act as those who are to be judged by the law of liberty (Jas 2:12)." What does this mean? Does it have something to do with the fact that we have been freed and brought into a close-knit, loving community where the ground is level before the cross? How should that affect our speech and actions? The context gives us the answer to this question.
"If a man comes into your assembly with a gold ring and dressed in fine clothes, and there also comes in a poor man in dirty clothes, and you pay special attention to the one who is wearing the fine clothes, and say, "You sit here in a good place," and you say to the poor man, "You stand over there, or sit down by my footstool," have you not ... become judges with evil motives?... you are committing sin and are convicted by the law as transgressors. (Jas 2:2-9)."
So speech, action, and heart are all connected. Evil motives affect speech and actions. But James says it can also work the other way around. "The tongue is set among our members as that which defiles the entire body, and sets on fire the course of our life, and is set on fire by hell (Jas 3:6)."
Does our heart defile our tongue, or does our tongue defile our heart? The answer is yes. If an impure thought comes into our mind, getting rid of it involves eradicating it, not letting it slip through your lips. If it comes out of your mouth, it defiles you.
As James says, we should be slow to speak. Since words are so powerful, we should carefully consider their effect before using them. Careless words spoken in anger, gossip, and coarse jesting can all start a blazing inferno that "defiles" us and "sets on fire the entire course of our life."